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Date: Wed, 29 Dec 2021 11:02:29 +0100
From: Markus Wichmann <nullplan@....net>
To: Rich Felker <dalias@...c.org>
Cc: musl@...ts.openwall.com
Subject: Re: ASM-to-C conversion for i386
On Mon, Dec 27, 2021 at 10:00:11AM -0500, Rich Felker wrote:
> On Sun, Dec 26, 2021 at 09:42:38PM +0100, Markus Wichmann wrote:
> > So I've converted __set_thread_area(). That was pretty straightforward
> > once I found SYSCALL_NO_TLS. The generated assembly generated by clang
> > 6.0.0 hits the same notes as the handwritten code, so I'm willing to
> > count that as a win.
>
> Somewhere I had an old version of this which didn't fare so well, so
> it will be interesting to compare.
>
Not quite sure what you mean by "didn't fare so well". I just fixed a
bug, and looked at the disassembly. Here's clang's output
(v6.0.0-ubuntu):
0005a482 <__set_thread_area>:
5a482: 53 push %ebx
5a483: 56 push %esi
5a484: 83 ec 10 sub $0x10,%esp
5a487: e8 00 00 00 00 call 5a48c <__set_thread_area+0xa>
5a48c: 59 pop %ecx
5a48d: 81 c1 74 2b 04 00 add $0x42b74,%ecx
5a493: 8b 44 24 1c mov 0x1c(%esp),%eax
5a497: 8b b1 74 02 00 00 mov 0x274(%ecx),%esi
5a49d: 89 e2 mov %esp,%edx
5a49f: 89 32 mov %esi,(%edx)
5a4a1: 89 42 04 mov %eax,0x4(%edx)
5a4a4: c7 42 08 ff ff 0f 00 movl $0xfffff,0x8(%edx)
5a4ab: c7 42 0c 51 00 00 00 movl $0x51,0xc(%edx)
5a4b2: b8 f3 00 00 00 mov $0xf3,%eax
5a4b7: 87 da xchg %ebx,%edx
5a4b9: cd 80 int $0x80
5a4bb: 87 da xchg %ebx,%edx
5a4bd: 85 c0 test %eax,%eax
5a4bf: 74 22 je 5a4e3 <__set_thread_area+0x61>
5a4c1: 31 f6 xor %esi,%esi
5a4c3: 46 inc %esi
5a4c4: 89 e1 mov %esp,%ecx
5a4c6: b8 7b 00 00 00 mov $0x7b,%eax
5a4cb: ba 10 00 00 00 mov $0x10,%edx
5a4d0: 89 f3 mov %esi,%ebx
5a4d2: cd 80 int $0x80
5a4d4: b9 07 00 00 00 mov $0x7,%ecx
5a4d9: 85 c0 test %eax,%eax
5a4db: 74 18 je 5a4f5 <__set_thread_area+0x73>
5a4dd: 89 c6 mov %eax,%esi
5a4df: 79 14 jns 5a4f5 <__set_thread_area+0x73>
5a4e1: eb 14 jmp 5a4f7 <__set_thread_area+0x75>
5a4e3: 8b 04 24 mov (%esp),%eax
5a4e6: 89 81 74 02 00 00 mov %eax,0x274(%ecx)
5a4ec: 8d 0c c5 03 00 00 00 lea 0x3(,%eax,8),%ecx
5a4f3: 31 f6 xor %esi,%esi
5a4f5: 8e e9 mov %ecx,%gs
5a4f7: 89 f0 mov %esi,%eax
5a4f9: 83 c4 10 add $0x10,%esp
5a4fc: 5e pop %esi
5a4fd: 5b pop %ebx
5a4fe: c3 ret
And here's gcc's (version 7.5.0-3ubuntu1):
00054908 <__set_thread_area>:
54908: 56 push %esi
54909: 53 push %ebx
5490a: e8 b0 3a fc ff call 183bf <__x86.get_pc_thunk.bx>
5490f: 81 c3 f1 16 04 00 add $0x416f1,%ebx
54915: 83 ec 10 sub $0x10,%esp
54918: 8b 83 fc 02 00 00 mov 0x2fc(%ebx),%eax
5491e: 89 e1 mov %esp,%ecx
54920: c7 44 24 08 ff ff 0f movl $0xfffff,0x8(%esp)
54927: 00
54928: c7 44 24 0c 51 00 00 movl $0x51,0xc(%esp)
5492f: 00
54930: 89 ca mov %ecx,%edx
54932: 89 04 24 mov %eax,(%esp)
54935: 8b 44 24 1c mov 0x1c(%esp),%eax
54939: 89 44 24 04 mov %eax,0x4(%esp)
5493d: b8 f3 00 00 00 mov $0xf3,%eax
54942: 87 da xchg %ebx,%edx
54944: cd 80 int $0x80
54946: 87 da xchg %ebx,%edx
54948: 85 c0 test %eax,%eax
5494a: 75 14 jne 54960 <__set_thread_area+0x58>
5494c: 8b 04 24 mov (%esp),%eax
5494f: 89 83 fc 02 00 00 mov %eax,0x2fc(%ebx)
54955: 8d 34 c5 03 00 00 00 lea 0x3(,%eax,8),%esi
5495c: 31 c0 xor %eax,%eax
5495e: eb 23 jmp 54983 <__set_thread_area+0x7b>
54960: b8 7b 00 00 00 mov $0x7b,%eax
54965: bb 01 00 00 00 mov $0x1,%ebx
5496a: ba 10 00 00 00 mov $0x10,%edx
5496f: cd 80 int $0x80
54971: 85 c0 test %eax,%eax
54973: 74 04 je 54979 <__set_thread_area+0x71>
54975: 78 0e js 54985 <__set_thread_area+0x7d>
54977: eb 0a jmp 54983 <__set_thread_area+0x7b>
54979: be 07 00 00 00 mov $0x7,%esi
5497e: b8 01 00 00 00 mov $0x1,%eax
54983: 8e ee mov %esi,%gs
54985: 83 c4 10 add $0x10,%esp
54988: 5b pop %ebx
54989: 5e pop %esi
5498a: c3 ret
So no, it's not the same as the hand-written assembler (but then, I
don't think there's a way to make a C compiler emit that code), but it
does hit the right notes. The system calls are called with the right
values, and the correct thing is moved into GS in the end. And isn't
that what counts?
Not quite sure why that is a 32-bit move, when I explicitly requested
the operand be a 16-bit value, but in this case it does not matter, as
the high sixteen bits are discarded in that instruction.
Ciao,
Markus
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